Increased power density is a continuing goal of modern power supply design. High power density is particularly crucial in applications wherein the allocated space for the power supply relative to the power output is restricted. In addition to being highly compact, the power supply must also be efficient to limit heat-creating power dissipation.
In low to medium level power applications (e.g., 30 to 800 watts), a forward converter topology is widely used. A DC/DC forward converter generally includes an isolation transformer, a switch on a primary side of the transformer, and a rectifier and output filter on a secondary side of the transformer. The switch, coupled in series with a primary winding of the transformer, converts an input DC voltage into an AC voltage. The transformer then transforms the AC voltage to another value and the rectifier generates therefrom a desired DC voltage that is filtered by the output filter at an output of the forward converter.
A practical concern regarding forward converters is that a magnetizing current of the transformer must be taken into consideration during the design of the converter. Otherwise, the magnetic energy stored in a core of the transformer by the magnetizing current may cause a failure in the converter. One approach of recovering the magnetic energy (to reduce the deleterious effects associated therewith) includes a reset topology wherein a tertiary reset winding is added to the transformer.
In a forward converter employing a typical reset topology, as the switch conducts, a transformer magnetizing current rises, storing magnetizing energy in a core of the transformer. During a complementary interval, the switch is turned off. During this period, the stored magnetizing energy is recovered and returned to the source of DC power via the tertiary reset winding and a diode (coupled thereto), thereby resetting the transformer.
One inherent problem with the reset topology is the maximum reset voltage for the transformer. At a 0.5 duty cycle for the switch, for instance, a drain-to-source voltage across the switch must be at least twice the input voltage. By operating the switch at less than the 0.5 duty cycle, the selection criteria for the switch and the turns ratio of the transformer are limited. Obviously, the aforementioned limitations detract from the use of forward converters and raise the costs associated therewith.
Accordingly, what is needed in the art is a system and method for recovering the magnetic energy stored in the transformer core and resetting the transformer while, at the same time, maximizing the design flexibility and increasing the efficiency of the power converter.